The world relies heavily on energy produced from fossil fuels, but as a non-renewable energy source (at least in our lifetimes), fossil fuels have serious limitations. The ever-increasing demand and dwindling supply of fossil fuels will inevitably cause significant problems in the future. In remote areas of developing countries, transmission and distribution of fossil fuel-generated energy can be difficult and expensive. Additionally, the burning of fossil fuels results in the formation of smog and global warming, and further contributes to our environmental problems. Thus, developing a clean alternative energy industry is key to improving the quality of life for individuals and communities, and to ameliorate global warming and other environmental problems.
The critical technical barrier to the widespread application of various alternative energy technologies is performance limitations of the key materials. For example, in solar-fuel production, the efficiency is relatively low since the current photocathode materials show sluggish H2 evolution reaction kinetics and the photoanode materials have insufficient light absorption and carrier collection capabilities. In wind power, the lifetime of turbine blades currently made of polymer-matrix composite materials reinforced with fiberglass or graphite fibers can be further enhanced when a new material with adequate stiffness to prevent failure as well as sufficient long term fatigue in harsh conditions is developed. Therefore, materials science and technology plays a pivotal role in building the world's energy future, from fundamental discovery science, to improving energy production processes. The discovery and optimization of new materials could effectively advance solutions to our energy challenges.
The demand for clean, secure, and renewable energy has stimulated great interest in fuel cells. Fuel cells are one distinct category of devices that are capable of converting chemical energy into electrical energy. Among the fuel cells that are currently under active development, alkaline, polymeric-electrolyte-membrane and phosphoric-acid fuel cells all require essentially pure hydrogen as the fuel to be fed to the anode.
Solid Oxide Fuel Cells (“SOFCs”), on the other hand, are a type of fuel cells that use a solid oxide or ceramic as the electrolyte of a cell. The basic solid oxide fuel cell is generally made up of three layers. A single cell consisting of these three layers stacked together is typically less than a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an “SOFC stack”. The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 500 to 1,000° C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again.
SOFCs offer great promise for the most efficient and cost-effective utilization of a wide variety of fuels such as hydrocarbons, coal gas and gasified biomass. Because of the relatively high operating temperature (500-1000° C.), the fuel processing reaction can be carried out within the cell stacks without additional fuel processors. Another advantage of SOFCs is the fuel flexibility. A wide variety of practical hydrocarbons such as methane, propane, gasoline, diesel and kerosene can be directly utilized as the fuels in SOFCs. The direct utilization of hydrocarbon fuels will increase the operating efficiency and reduce system costs, which will accelerate substantially the use of SOFCs in transportation, residential and distributed-power application. Among the hydrocarbon fuels, natural gas such as methane is regarded as relatively cheap and popularly available fuel with plenty of deposits. Additionally, SOFCs that can directly run on natural gas would highly reduce the operating cost and accelerate the commercialization of SOFC system.
The basic chemical reactions at the anode side of an SOFC is the oxidation of fuels, such as hydrogen gas and/or carbon monoxide, to generate electrons:Anode: H2+O2−→H2O+2e−and/or CO+O2−→CO2+2e−
The reaction at the cathode side is the reduction of oxygen to oxygen ions:Cathode: O2+4e−→2O2−
Therefore, the overall reaction of an SOFC becomes:Overall: H2+½O2→H2OOr H2+CO+O2→H2O+CO2 
Therefore, SOFCs typically run on pure hydrogen or mixture of hydrogen and carbon monoxide by internally or externally reforming a hydrocarbon fuel, while air serves as the oxidant. As shown above, if pure hydrogen is used, then the product is pure water, whereas carbon dioxide is produced if carbon monoxide is also used. Carbon dioxide and water are produced is hydrocarbon fuels are used.
In an effort to reduce fuel electrode manufacturing costs, sintering processes have been attempted, such as those described in U.S. Pat. Nos. 4,971,830, 5,035,962, 5,908,713 and 6,248,468. However, fuel electrodes applied by a sintering process are relatively time consuming in that it still requires at least two processing steps, an initial application followed by high temperature sintering. Moreover, sintered fuel electrodes may experience marginal physical stability over time.
Other attempts to reduce fuel electrode fabrication costs include plasma spraying (e.g. atmospheric plasma spraying “APS”, vacuum plasma spraying “VPS”, plasma arc spraying, flame spraying) which generally involves spraying a molten powdered metal or metal oxide onto an underlying substrate surface using a plasma thermal spray gun to form a deposited layer having a microstructure generally characterized by accumulated molten particle splats. Plasma spraying techniques are described in U.S. Pat. Nos. 3,220,068, 3,839,618, 4,049,841, and 3,823,302 and 4,609,562 generally teach plasma spray guns and use thereof, each of which are herein incorporated by reference in their entirety. Although plasma spraying has been used for fabrication of certain fuel cell layers, such as those described in U.S. Pat. Nos. 5,085,742, 5,085,742, 5,234,722 5,527,633 (plasma sprayed electrolyte) U.S. Pat. No. 5,426,003 (plasma sprayed interconnect), U.S. Pat. No. 5,516,597 (plasma sprayed interlayer) and U.S. Pat. No. 5,716,422 (plasma sprayed air electrode), use of such plasma spraying techniques have been of limited value when used to apply a fuel electrode onto an electrolyte because they tend to result in a fuel electrode that poorly adheres to the electrolyte and exhibits poor thermal cyclability due to the mismatch of thermal coefficients of expansion between the metal portion of the fuel electrode and the ceramic electrolyte. Moreover, these conventional plasma spraying techniques tends to result in a fuel electrode that has a low porosity after continued use, thereby causing voltage loss when current flows as a result of polarization due to a low rate of diffusion of fuel gases into and reaction product out from the interface between the fuel electrode and electrolyte.
There is thus a need for a SOFC and a method for making the SOFC that can generally achieve above-described favorable technical properties and can be manufactured at a low cost.